Process for the manufacture of cast iron having an eutectic graphite formation



United States Patent 3,133,813 PROCESS FOR THE MANUFACTURE OF CAST RON HAVING AN EUTECTIC GRAPHITE FORMATION Wolfram Ruff, Frankfurt am Main, Germany, assignor to Alfred Teves lvlaschiinlenund Armaturenfabrik KG, Frankfurt am Main, Germany No Drawing. Filed June 2, 1961, Ser. No. 126,385 Claims priority, application Germany Aug. 30, 1960 2 Claims. (CI. 75-53) The invention relates to the production of a cast iron having an eutectic graphite formation, and, more particularly, containing exclusively a formation known as D- graphite. This type of cast iron is of especial value in the manufacture of grey-setting piston rings, e.g., for use in the automotive industry, which subsequently are heated and tempered and which must be cast with a very fine graphite formation in order to insure great strength, as required in such piston rings.

As is well known, U.S. standards differentiate between five types of graphitic formation in grey-setting cast iron which are named types A to E. This invention pertains solely to the production of the D-type, an interdendritic segregation of graphite flakes of random orientation. Hardly any metallurgical directions are in existence for the planned manufacture of this special type D graphite flakes because to date it had generally been assumed that this type D should be avoided in grey-setting cast iron in order to protect the cast iron from a deterioration in its physical properties. Hence, the directions primarily relate to manners of avoiding a D-type formation, for instance by addition of alloying materials or also by special inoculation with a variety of additives after melting of the cast iron in the ladle. The instant invention also varies in this respect from the prior art by the special feature of a particular treatment which is carried out during the melting of the cast iron in the furnace, as will be set forth below.

Therefore, the purpose of the invention, in contrast to measures avoiding the formation of D-graphite, relates especially to the production of this particular type, because it now has been established unexpectedly that, especially in tempered cast iron which contains D-graphite, the physical properties improve considerably over cast iron containing other types of graphite.

In order to assure attainment of these physical improvements in large-scale production, a novel process has been devised. This process starts with a white-setting cast iron, whereby the melt, adjusted to white or mottled setting, is subjected to a post-treatment in two successive stages.

These second and third melt treatment steps consist of (a) a short-time post-treatment with a small quantity of flake graphite, and (b) a further treatment, after removal of the flake graphite, with a slag free of carbon, consisting preferably of melting glass pieces or of core sand which is strewn on the surface of the melt. This slag, after removal of the flake graphite, again must cover the melt thoroughly. This third step, in the course of intensive investigations, has been found urgently required for the purpose at hand, because the solidification of the melt after the second step, i.e., the treatment with flake graphite, merely leads to an irregular graphite formation within the structure. This irregular formation manifests itself by the simultaneous presence of undesirable very coarse and undesirable very fine graphite emplacements next to each other.

The following is given as an exemplification for the preparation of a cast iron containing exclusively D- graphite. However, it should be understood that this is given merely by way of illustration, not of limitation, and that numerous changes may be made in the details without departing from the spirit and the scope of the inven tion as hereinafter claimed.

The material Was a melt obtained in an oven heated by electrical induction and having a working capacity of 600 kg. The melt designed for tempered piston rings and manufactured by sand-casting, had a bending strength of 70-95 kg./mm. Without the primary D-graphite separating from the melt, at final Brinnelhardn'esses of 285- 325 kg./mm. bending strengths of such magnitude had not been obtained in piston rings heretofore.

The charge of the 600 kg. melt for the induction oven had been adjusted to show the following analysis:

In order to test the graphite formation of the liquid iron, small quench samples were withdrawn and broken. These samples showed, however, that the iron in the form then present was not yet usable because of its insuflicient tendency toward graphite formation, i.e., because, under these conditions, an overly strong white setting would occur.

In accordance with the prevailing opinions and with the prior art, a given amount of ferrosilicon or of another silicon-containing inoculating material would have been all that were needed to increase the tendency of the liquid iron toward primary graphite separation. However, experiences gained over a period of many years have shown that such a silicon addition does not have the desired effect in the instant case since the silicon, when added only in small quantities, in a 600 kg. melt in an induction oven rapidly becomes lost and, when added in larger amounts, causes such an increase in the graphite formation that no uniform D-graphite forms in the piston rings. Piston rings with a uniform D-graphite formation could solely be obtained from the melt described when, after melting and pre-testing, the slag was removed completely, and the post-treatment, first with flake graphite and then with a final slag, was carried out.

To cover the 600 kg. melt, only approximately 1-2 kg. flake graphite were required which were applied for approximately 10 minutes at a melt temperature of approximately 1,500l,550 C., whereafter the remaining flake graphite was cleanly skimmed olf the melt before applying the final slag.

The latter consisted of simple dry core sand with which the melt was covered for 10 minutes at the same temperature (approximately 1,500-1,550 C.). Without this last step, the D-graphite desired never could be obtained satisfactorily but, instead, very irregular structures formed. However, with this final slag treatment, the desired effect was obtained easily and regularly so that a large-scale production method for the piston rings named above containing solely D-graphite was established.

The advantages gained by the new melting process described above are manifold and consist especially in the following:

(1) A way has been found to obtain an extremely stable and well adjustable melt condition in lieu of the former unstable and diflicultly controllable melt condition of a cast iron setting white or mottled; and

(2) Flake graphite disposed on a bath of metal alone leads to a much more rapid graphitizability of the iron than can be attained, for instance, by addition of a high percentage of ferrosilicon to the bath whose silicon, as a rule, yields merely an initial temporary advantage, but no lasting effect because it dissolves much more rapidly in the melt than an added flake graphite. Too high a silicon addition, moreover, easily leads to coarse graphite separation, not to fine eutectic graphite formations, and the latter alone cause the described strength properties of the cast iron.

Aside from a silicon content as low as possible with highest speeds of graphite formation in an iron, it, naturally, also is desirable to have these highest speeds of graphite formation take place simultaneously at lowest carbon content of the iron, whereby it has been established that the above-described special treatment of the liquid iron with flake graphite between two slag treatment is the most successful manner of obtaining an exclusive formation of D-graphite.

I claim as my invention:

1. A process for the manufacture of cast irons having an eutectic D-type graphite formation, which comprises slagging a white-setting iron melt, covering said melt with flake graphite, skimming off said fiake graphite and covering the melt with a carbon-free slag.

2. A process for the manufacture of cast irons for use in piston rings of high bending strength, said cast irons having an eutectic D-type graphite formation, which comprises preparing a white-setting iron melt in a furnace, slagging said melt, covering said melt for approximately 10 minutes with flake graphite at approximately 1,500- 1,550 C., skimming off said flake graphite, covering said melt for approximately 10 minutes at the same temperature with a carbon-free slag, selected from the group consisting of melting glass and core sand, removing the slag and pouring the melt into the molds.

No references cited. 

1. A PROCESS FOR THE MANFACTURE OF CAST IRONS HAVING AN EUTECTIC D-TYPE GRAPHITE FORMATION, WHICH COMPRISES SLAGGING A WHITE-SETTING IRON MELT, COVERING SAID MELT WITH FLAKE GRAPHITE, SKIMMING OFF SAID FLAKE GRAPHITE AND COVERING THE MELT WITH A CARBON-FREE SLAG. 